N-Fluoropyridinium salts including N-fluoropyridinium trifluoromethanesulfonate have previously been reported as useful fluorinating agents. (Tetrahedron Lett., 27, 4465 (1986) and Japanese Patent Laid-Open No. 63-10764.) However, the use of such fluorinating agents does not always provide satisfactory selectivity with regard to the reaction positions in the product.
For example, when phenol is fluorinated using N-fluoropyridinium trifluoromethanesulfonate, three components of o-fluorophenol, p-fluorophenol and 2,4-difluorophenol are produced. (Japanese Patent Laid-Open No. 63-10764, Reference Example 1). When this compound is used to fluorinate a trimethylsilylenol ether derivative of a steroid, not only the 6-fluorosteroid (which is fluorinated at the 6-position and which is important as a physiologically active substance) but also the 4-fluorosteroid (which is fluorinated at the 4-position) are produced in a ratio of about 2.3:1. (Japanese Patent Laid-Open No. 63-10764, Reference Example 37).
Since the process for separating and purifying each of the isomers is complicated, the yield and overall efficiency of the process is low. Production methods which produce many isomers are generally undesirable.
N-Fluoro-6-chloropyridinium-2-sulfonate has also been reported as a fluorinating agent which has excellent selectivity for a fluorination position. (Advance Notes of the 14th Meeting of Fluorine Chemistry (Japan), 1989). However, there is a problem in that 6-chloropyridine-2-sulfonic acid, which is a starting material in the synthesis of the fluorinating agent, cannot be produced in a sufficiently high yield.